This invention relates to a bias circuit for an avalanche photo-diode which is employed as a light receiving element in a light receiving device in which light receiving and pause times occur repeatedly.
An avalanche photo-diode (hereinafter referred to merely as an "APD") operates to amplify, with a bias voltage, a photoelectric current I.sub.P obtained through photo-electric conversion, and the current amplification factor .mu. representing the degree of amplification thereof is, in general, expressed by the following equation: ##EQU1##
where V.sub.A is the bias voltage, V.sub.B is the breakdown voltage, and n is a constant between one and three, and where V.sub.A &lt;V.sub.B.
It is essential for the light receiving device to have a stable current amplification factor .mu. excellent in S/N ratio. Furthermore, as the breakdown voltage V.sub.B has a positive temperature coefficient, it is necessary to maintain the factor .mu. constant against the variation of the breakdown voltage V.sub.B and to obtain a bias voltage V.sub.A which is stable. A light receiving device in which light receiving and pause times occur repeatedly may employ a method in which the APD breakdown voltage V.sub.B is detected by utilization of the pause time, and where the bias voltage V.sub.A in the light receiving time is set according to the voltage V.sub.B thus detected. In setting the bias voltage V.sub.A according to this method, it is essential that the variation in the breakdown voltage V.sub.B of the avalanche photo-diodes and the variation in the temperature characteristic of the breakdown voltages V.sub.B be corrected to thereby set the bias voltage with which the current amplification factor .mu. is made constant. In addition, it is necessary for the circuitry for this function to be simple, and the number of means for adjusting the fluctuating characteristics of the avalanche photo-diodes be minimized.